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First, the car needs a certain voltage at a certain current to operate. as an aside:
Generators do not need a battery as they store residual magnetism and use that to generate current, however a starter draws more than five to ten times auto generators' capacities. Thus "popping the clutch" gives you electrical current with a generator which has a voltage regulator to regulate both voltage and current. Alternators need a very small excitation voltage, not magnetism, so without a battery there is not voltage. No battery to excite the winding, no output. Alternators if disconnected or dumped while producing electrical power can cause spikes up to around 60 Volts DC. In general, an alternator equipped car should only be run with a battery, even with a manual trans.
On the question,usually the car's voltage is dropping below the minimum to operate, on computer cars it's about 9 to 10 volts. So if you step on the brakes- the brake light current draw , or turn on headlights-the headlight draw, etc. cause the current to divide lowering the voltage and causing the car to die.
On the question, if not a bad battery, this can be caused by a high resistance or bad connection. Get the schematics and check all connections from the battery to the ignition and any fuel pumps up to the branch connection for the things that make it die-those branch connections are good else they could not flow enough current to drop the voltage. Included in the words bad connection are also bad crimps and failed insulation.
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